These charges accumulate on the surface of the car due to various causes, such as ionisation of the air caused by lightning, friction due to movement of the car through the air, friction of the various belts in the engine pulley grooves, lack of humidity in the air, CO2 from combustion, electrical fields etc.
These charges accumulate also in persons due to friction of synthetic underwear against the body and rubbing against the car seats; said charges reach voltage values of several tens of thousands of volts.
The phenomenon is perceived when the person puts his feet on the ground and at the same time pushes the car door to close it.
At present, a device commonly sold on the market consists of a flexible strip of conducting rubber earthed to the bodywork at one end with the other end trailing along the ground.
This system does not always work, however, for various reasons.